1. Field of the Invention
This invention relates to a navigation device which supports driving of a vehicle such as a car or the like by displaying and informing current position and travel direction of the vehicle, and more particularly to an offset processing of the navigation device for correcting an angular velocity data measured by an angular velocity sensor such as a gyro.
2. Description of Prior Art
A navigation device installed in a vehicle such as a car or the like has been required to precisely measure current position and travel direction of the vehicle and then accurately and rapidly display the current position, the travel direction of the vehicle and other information for satisfying user's demands together with a map information corresponding to the current position for assistance of a driver.
One type of navigation devices calculates the current position and travel direction of the vehicle from electric waves transmitted from a GPS (Global Positioning System) satellite (hereinafter referred to as "GPS measurement method"). Another type of navigation device calculates them from data of a direction sensor such as a gyro, a geomagnetic sensor or the like and a travel sensor for measuring a travel distance or the like, each installed in the vehicle (hereinafter referred to as "stand-alone type measurement method"). The GPS measurement method has such various advantages that it is not required to set in advance a vehicle position on a map and that the measurement error of the vehicle position is remarkably small and accordingly high reliability can be achieved. Therefore, the GPS measurement method is broadly used. However, the GPS measurement method can not used when the vehicle travels through a shade of buildings, woods or in a tunnel. On the other hand, the stand-alone type measurement method has such disadvantage that the measured data is not always accurate because of the accumulated error, influences from variations of temperature or condition in the vehicle, and bad influence on the geomagnetic sensor resulting from traveling on iron bridges. Accordingly, the two types of measurement methods are used in combination in order to make up for the above deficiencies with each other.
Since output data of an angular velocity sensor used for the stand-alone type measurement, such as a gyro, varies according to the variation of temperature or humidity, the output data is corrected by using an offset value which is occasionally reset (renewed).
However, the above angular velocity correction method can not accurately detect the change of the direction when the vehicle is rotated by a turntable or the like in a parking zone, for example. This is because, in such a case, since the vehicle is stopped even though the turntable is rotated, the reset (renewal) of the offset value is erroneously executed.